As a battery pack that has a battery cell has been known a battery pack described in, for example, JP 2009-211829A. The battery pack described in JP 2009-211829A circulates fluid in a housing in which a plurality of battery cells are housed to thereby deprive the plurality of battery cells of heat, thereby being capable of cooling the heated battery cells.
In the battery pack described in JP 2009-211829A, the fluid once circulated deprives the battery cells of heat in a process in which the fluid is again circulated, so that the fluid having once received the heat from the battery cells needs to radiate the heat to an inside wall surface of the housing. For example, like a comparative example shown in FIG. 10, by forming fins 10 in the whole region of a fluid passage in parallel to a flow direction of the fluid, a heat radiation area between the fluid flowing along the inside wall surface and the inside wall surface can be increased. Hereinafter, a construction shown in FIG. 10 is referred to as “a comparative example”. Here, a heat radiation amount to the inside wall surface from the fluid is proportional not only to the heat radiation area but also to a temperature difference between the fluid and the outside air. In this point, in a region C shown in FIG. 10, the temperature difference between the fluid and the inside wall surface is sufficiently large and hence the heat radiation amount is lame. However, since the fluid is deprived of heat by the fins 10 in a process in which the fluid flows between the fins 10, in a region D, the temperature difference between the fluid and the outside air becomes small and hence a sufficient heat radiation amount by the fins 10 cannot be secured. In other words, although the fins 10 in the region D have as large a heat radiation area as the fins 10 in the region C, a difference is caused in the heat radiation amount between the fins 10 in the region D and the fins 10 in the region C, which hence reduces a heat radiation efficiency in the fins 10 in the region D.